Add create/delete project observation endpoints from v1 API

[JWCook]

API reference: https://api.inaturalist.org/v1/docs/#!/Projects/post_projects_id_add

• POST /projects/{id}/add
• DELETE /projects/{id}/remove
• Doctrings + type annotations
• Usage examples
• Sample response data
• Unit tests

TODO: I'm not quite sure what the difference is between these two endpoints and POST/DELETE /project_observations.

• Both `POST endpoints have the same description of "Add an observation to a project."
• DELETE /projects/{id}/remove: "Remove an observation from a project"
• DELETE /project_observations/{id}: "Delete a project observation"

[AugustT]

+1 for adding observations to projects. For tool I'm developing I would like observations to automatically be added to projects. At the moment that has to be done manually afterwards.

[JWCook]

@AugustT
add_project_observation() is now available in 0.15.0dev0

[AugustT]

Sweet